Interfacial Challenge for Solid-State Lithium Batteries- Liquid Addition

Abstract

All solid-state lithium batteries with garnet electrolytes (Li7La3Zr2O12, LLZO) are promising energy storage devices that have gained increasing attention due to their huge potential towards non-flammability and higher energy density. However, reported solid-state lithium batteries cannot achieve the projected energy density (> 500 Wh/kg at the cell level) mainly due to insufficient contact and poor compatibility between LLZO and electrodes. The use of liquid electrolytes in small quantities has been suggested as a component in the quasi-solid hybrid electrolytes to address these two issues. However, the working principle of liquid electrolytes added as the interface inside the batteries is not clear yet.This study added 10L carbonate-based liquid electrolyte between LLZO and LiNi0.6Mn0.6Co0.2O2 (NMC 622) cathode. The assembled Li|LLZO|NMC 622 cell exhibited an initial discharge capacity of 168 mAh g-1 with a capacity retention ratio of ~82 % after 28 cycles. Scanning Transmission X-ray Microscopy revealed the reaction of LE with garnet and NMC 622. More importantly, the LE decomposed and solidified during the cycling process. Decomposed LE participated in the formation of a newly-identified solid-liquid electrolyte interface (SLEI) just after the 1st cycle. Furthermore, the X-ray Absorption Spectroscopy results indicated that the SLEI consisted predominantly of LiF, LaF3, Li2O, and Li2CO3 species.Overall, this study proved the solidification of liquid electrolytes at the garnet/cathode interface. The formation of SLEI effectively suppressed the degradation of the garnet electrolyte and stabilized the battery cycling performance. More efforts are required to optimize the liquid and establish a more stable SLEI that could expand the cycling life of the batteries.